Protein kinases are enzymatic components of the signal transduction pathways which catalyze the transfer of the terminal phosphate from ATP to the hydroxy group of tyrosine, serine and/or threonine residues of proteins. The overexpression or inappropriate expression of normal or mutant protein kinases in mammals has been a topic of extensive study and has been demonstrated to play a significant role in the development of many diseases, including diabetes, angiogenesis, psoriasis, restenosis, ocular diseases, schizophrenia, rheumatoid arthritis, atherosclerosis, cardiovascular disease and cancer. In summary, inhibitors of protein kinases have particular utility in the treatment of human and animal disease.
FLT3 (Fms-like tyrosine kinase 3), together with c-Kit, c-FMS and PDGFR, belongs to members of receptor tyrosine kinase III (RTK III) family, the protein structure of which include an extracellular region consisting of five immunoglobulin (Ig)-like domains, a transmembrane region, an intracellular juxtamembrane (JM) region, as well as two tyrosine kinase (TK) domains interrupted by a kinase insert in the intracellular region (S. D. Lyman et al., Oncogene, 1993, 8, 815-822). In 1996, the FLT3 mutations were first identified in AML cells, and the mutation type was internal tandem duplications (FLT3/ITD). In recent years, many studies have demonstrated that the FLT3-actived mutations play a very important pathological role in the development of AML and the progression of the disease. AML patients with the FLT3/ITD-actived mutations normally have unique clinical features, such as high peripheral blood leucocyte count, poor clinical prognosis, and easy relapse, and the like. As the method for detecting the FLT3-actived mutations is simple and practicable, more and more researchers are committed to developing FLT3 as a conventional detection means of AML for guiding the therapy and prognostic prediction in AML patients, and as a detection means of minimal residual leukemia, and as a new target for chemotherapy in leukemia patients.
Hematological malignancies are cancers of the body's blood forming and immune systems, the bone marrow and lymphatic tissues. Whereas in normal bone marrow, FLT3 expression is restricted to early progenitor cells, in hematological malignancies, FLT3 is expressed at high levels or FLT3 mutations cause an uncontrolled induction of the FLT3 receptor and downstream molecular pathway, possibly Ras activation. Hematological malignancies include leukemias, lymphomas (non-Hodgkin's lymphoma), Hodgkin's disease (also called Hodgkin's lymphoma), and myeloma—for instance, acute lymphocytic leukemia (ALL), acute myeloid leukemia (AML), acute promyelocytic leukemia (APL), chronic lymphocytic leukemia (CLL), chronic myeloid leukemia (CML), chronic neutrophilic leukemia (CNL), acute undifferentiated leukemia (AUL), anaplastic large-cell lymphoma (ALCL), adult T-cell ALL, AML with trilineage myelodysplasia (AML/TMDS), mixed lineage leukemia (MLL), myelodysplasia syndromes (MDSs), myeloproliferative disorders (MPD), multiple myeloma (MM) and myeloid sarcoma (Kottaridis, P. D., R. E. Gale et al., FLT3 mutations and leukaemia, British Journal of Haematology, 2003, 122(4):523-38; Ansari-Lari, Ali et al., FLT3 mutations in myeloid sarcoma, British Journal of Haematology, 2004, 126(6):785-91.).
It has been confirmed that there are mainly two classes of FLT3-actived mutations: Internal tandem duplications (ITD) and point mutation in the activation loop (TKD point mutation). Both classes of FLT3-actived mutations can cause spontaneous phosphorylation of FLT3, which leads to ligand-independent constitutive activation of FLT3, which further activates its downstream abnormal signal transduction, thereby acing to promote proliferation and inhibit apoptosis, so that the leukemia patients with the mutant phenotype have poor clinical prognosis.
At present, the targeting inhibition of FLT3 and mutant FLT3 has become a hotspot. It is mainly for the development of small molecules as tyrosine kinase inhibitors, which inhibit the kinase activity by competing with FLT3 tyrosine kinase for ATP binding sites. The inhibitors of FLT3 kinase, such as AC220, have now been introduced into clinical practice.